The present invention relates to megasonic cleaning systems for cleaning semiconductor wafers and other items which require the removal of extremely small particles. Such systems are generally comprised of a tank, for holding cleaning fluid and articles to be cleaned, and piezo electric transducers for transmitting acoustic energy at a frequency of approximately 1000 kilohertz into the cleaning tank. Typically, the transducers are mounted either in the interior of the tank or on the exterior of the tank. A disadvantage of these arrangements is that they do not allow a set of transducers to be easily used in conjunction with a number of different cleaning tanks.
A further disadvantage of the prior megasonic cleaning systems is that they do not provide high levels of acoustical impedance matching between the transducers and the cleaning fluid. This does not allow for maximum energy transfer between the transducers and the cleaning liquid. In this regard, it is noted that U.S. patent application Ser. No. 08/139,981, filed Sep. 28, 1993, (hereinafter "the '981 application"), of which the inventor of the present application is a coinventor, discloses the use of a transducer mounted in a quartz tube which is in turn mounted inside a cleaning tank. The quartz tube is a quarter wavelength thick and acts as an impedance matching layer between the transducer and the cleaning fluid in the tank. Quartz, however, is not an optimal impedance matching layer. The invention disclosed in the '981 application, because it has been on sale for more than a year, is prior art to the invention of the present application. The disclosure of the '981 application is hereby incorporated in the present application by reference.
A still further disadvantage of the prior art is that it does not provide a method for monitoring the operation of the transducers.